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It's Not All Torque

The inside story on the Miller-cycle engine

The name for the Miller-cycle engine comes from an American engineer, Mr Ralph Miller who patented his version of the forced induction Otto-cycle in the 1940s.

Until now his principle had only been used in low engine speed applications - such as driving big ships and also for power generation by stationary engines.

The engine in the Mazda Millenia is a 2.3 litre, quad cam V6 which is designed to perform better than a larger 3.0 litre engine but with the efficiency of a smaller (2.0L) unit.

It provides the driver with high performance coupled with between 10 and 15 percent less fuel consumption.

Power and torque figures are: 164kW of power @ 5.500rpm and 294Nm of torque @3,500 rpm. This compares with 149kW @6500 and 223Nm@4800 for the base 2.5 litre vehicle equipped with the conventional Otto-cycle engine.

From the outside, the Miller engine looks similar to other hi-tech units. Aluminium block, lots of belts, 24 valves, four camshafts, except for the two intercoolers and a belt driven Lysholm compressor tucked neatly into the "Vee" between the cylinder banks.

So, how does this 2.3 litre engine produce more power and torque using less fuel than a larger engine, without many of the expected disadvantages; such as high emissions and engine knock?


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In simple terms, the compression stroke of the Miller-cycle engine is shortened with results in a low compression ration, yet a high expansion ratio.

In order to grasp this and other aspects of the Miller-cycle, one has to go back and have a look at some of the basic principles of internal combustion engine operation. There are four areas worth reviewing.

Engine Size vs Frictional Losses

When the displacement of an engine is reduced, there is a substantial reduction in frictional losses. For example, 25 percent less friction is produced rotating a particular engine that has its displacement reduced by 30 percent. An automatic offshoot of such downsizing is an improvement in fuel efficiency of around 10-15 percent.

Theoretical vs Actual Compression Ratio

The theoretical compression ratio is simply a comparison of the volume above the piston when it is at bottom dead centre (BDC), to the volume above it at top dead centre (TDC). However, in practice, the actual compression ratio is determined by the valve timing, since the real compression stroke does not begin until the intake valve closes. Similarly, the length of the power (expansion) stroke is also determined by the opening point of the exhaust valve.

With the fairly symmetrical valve timing being found in most engines these days, these two strokes are approximately the same. This means that the actual compression stroke is roughly equal to the expansion stroke.

Thermal Efficiency

By increasing the compression ratio, the thermal efficiency of an engine is also increased. However, along with this efficiency gain comes higher combustion pressures and temperatures. These characteristics are usually accompanied by two well known "bad guys" Oxide of Nitrogen (NOx) emissions and knock.

NOx is produced as a result of combustion pressures and temperatures greater than 1,300 deg C. At these temperatures the normally inert Nitrogen (78 percent by volume of intake air), reacts with oxygen to form oxides (nitrogen dioxide and nitrogen monoxide).

Knock is caused when part of the air/fuel charge is ignited spontaneously by the effect of heat and pressure and not the spark plug as Otto intended. This produces two flame fronts in the combustion chamber which can result in serious engine damage.

There are two important things to note here. Firstly, knock is affected by the gas temperature at TDC of the compression stroke. Secondly, most of the gain in thermal efficiency from increases in compression comes mainly from the events that occur on the expansion stroke (more push on the piston). Only a little is gained from the actual increase in compression ratio.

Pumping Losses

This refers to the energy required to rotate an engine during two of the three non-power producing strokes

- pumping air in and pumping exhaust gases out (but does not include frictional losses). It is a term that describes the efficiency of intaking and exhausting the charge. If the piston does less work in taking and exhausting, less power robbing pumping losses are produced.

One of the reason the original Otto-cycle had the exhaust valve opening brought forward (before BDC) is to allow the residual exhaust gas pressure (which, once the piston is half way down the power stroke is too low to provide much push on the piston) to expel itself and not have to rely on the piston to pump it all out, creating further pumping loss. This modified (Otto) valve timing allows around 50 percent of the exhaust gases to be expelled "for free" (no pumping losses incurred in getting rid of half of the exhaust gas). A throttled engine (eg cruising with high manifold vacuum) has high pumping losses since a vacuum is not produced for free; energy is consumed in doing so. Some experimental variable displacement engines reduce the number of working cylinders (switching some off by holding the valves open) under partload to reduce manifold vacuum and therefore pumping losses.

Volumetric Efficiency

The term volumetric efficiency refers to the ability of an engine to fill its cylinders with a volume of air equal to their displacement (100 percent Ve). The greater the Ve then the greater will be the output of that engine. Engine manufacturers go to great lengths to "tune" their engine design and obtain the greatest Ve. This involves a lot of research into gas flow - including manifold and port design - as well as valve timing and lift, together with multiple valves and combustion chamber design.

The easiest way to make dramatic improvements in Ve is to add an external device such as a supercharger or turbocharger. Its job is to "force feed" as much air as possible into each cylinder. But, as with increased compression ratio, excessively high combustion pressures and temperatures may be produced by forced induction. These can work against our intent to produce a powerful but clean engine.

The most common method to overcome this problem is to use an intercooler (as well as lowering the compression ratio). An intercooler is an air-to-air heat exchanger that has the ability to reduce air intake temperature (after the supercharger) by at least 50 deg C. This helps keep combustion temperatures to a safe level.

The modern internal combustion engine is a finely balanced mixture of all these (and many more) conflicting requirements.

Miller-cycle Technical Details

There are basically four means that the Miller-cycle uses to obtain its increased efficiency.

  • Smaller engine (lower displacement)
  • reduced compression stroke and pumping losses
(from late closing of the intake valve)
  • cooler intake charge (intercooled air)
  • combustion improvements
Small Engine

The graph below indicates the fuel efficiency increase as displacement is decreased. The horizontal axis begins at 1.0 which compares to a 3.3L's fuel efficiency, whilst 0.7 indicates a 30 percent reduction in displacement (down to 2.3L). The two curves represent the changes in efficiency gain with load changes (the greatest being at 20 percent load).


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An engine that has a lower compression ratio will also naturally produce smaller amounts of friction, particularly on the compression stroke. Since the Miler engine is targeted at a vehicle that would normally use an engine over 3.0L, the reduction in size to 2.3L provides an improvement in fuel efficiency of around 13 percent.

Reduced Compression Stroke Retaining High Expansion Stroke

At first glance the compression ratio would appear to be 10:1 (swept volume compared to clearance volume), however, for the first 20 percent of the compression stroke, the intake valves remain open. Since the actual compression stroke does not begin until the valve closes, the compression ratio is "artificially" reduced down to 8:1.


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Intake valve duration is from two degrees before TDC until 70 degrees after BDC, while the exhaust valve duration is from 47 degrees before BDC to five degrees after TDC. The intake valves remain open for around an additional 30 degrees of crankshaft rotation beyond "normal". This kind of valve timing reduces the effective compression ratio from 10:1 to a little under 8:1.

Unusual is the fact that the compression stroke is reduced but the power or expansion stroke remains the same. This is one of the critical points of difference from the Otto-cycle engine where the relationship between the expansion and compression is the same.

The late closing of the intake valve eliminates the substantial amount of energy normally required to overcome friction (as well as pumping losses), in the process of completing a normal compression stroke.

While this sounds good in theory, the usual result of blowing half the intake charge back out the intake valves would be a reduction in volumetric efficiency.

In the Miller-cycle engine, however, this is where the compressor comes to the rescue. Any loss of intake charge through "back flow" is more than compensated for by the density of the charge provided by the compressor. Under these circumstances, the Lysholm compressor is more efficient (lower pumping loss) at carrying out the job of filling the cylinders than a reciprocating piston.

The highly efficient Lysholm compressor consists of a male and female rotor, with three and five lobes respectively. Rotor speeds are up to 35,000rpm for the male and 21,000rpm for the female. Maximum discharge pressure is up to 150kPA. Advantages of the belt driven compressor include no lag, non-contacting rotors and none of the temperature extremes associated with turbocharger operation.

Cool Intake Charge

Due to the late closing of the intake valves (reduced compression ratio), less heat is added to the intake charge by the piston during this stroke. The loss in thermal efficiency of reduced compression ratio from 10 to 8:1 is only about six percent.

This slight loss in thermal efficiency from the decrease in compression ratio is more than made up for by a much denser charge supplied by the compressor. Cool dense air is pushed through twin intercoolers into the cylinders. This reduces the combustion chamber temperature at TDC of the compression stroke and so lowers the potential for detonation to occur and also production of NOx.

The end result of this delicate balance of valve timing, compression ratio and forced induction, is a cylinder that is well filled with cool dense air but at a lower cost in terms of energy consumption than a conventional four cycle would allow.

Combustion Improvements

For many years, swirl and squish were commonly used terms to describe the in-cylinder events affecting the rate and other characteristics of combustion. In more recent years, extensive study of vertical in-cylinder swirl, called "tumble" has been carried out.

On the Miller engine, the intake port has been shortened to promote smooth but strong intake air flow. A mask is added to the intake side of the combustion chamber to concentrate the air flow to the centre of the cylinder; strengthening the tumble motion.

Tumble promotes more ideal intake dynamics and combustion events that enhances the anti-knocking performance of the engine.

Conclusion

This engine utilises well proven conventional technology, but further enhances it to take into account growing international concerns for the environment and resource preservation.

While, in the fullness of time, engines which use alternative forms of energy may come to pass, Miller-cycle technology will be seen to have advanced the cause of efficiency and responsibility.



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